WARSAW (Reuters) - U.S.-owned private Polish broadcaster TVN said it was facing intimidation after members of Poland’s internal security agency entered a TVN cameraman’s house and called him to a hearing over accusations he propagated Nazi propaganda.

The camera operator, Piotr Wacowski, went undercover to film a neo-nazi group for a report broadcast in January by the network, which is controlled by U.S. entertainment firm Discovery Inc..

Poland’s National Prosecutor’s Office said on Sunday it was premature to accuse Wacowski and it had referred the case to a regional prosecutor’s office for further examination, Polish state-news agency PAP reported.

A spokesman for Poland’s internal security agency declined to comment. A government spokeswoman did not immediately respond to requests for comment.

Earlier this week, U.S. ambassador to Poland Georgette Mosbacher told Polish lawmakers that U.S. Congress could withdraw support for Poland if the free press faced attacks, Polish news website Polska Times reported.

The U.S. embassy in Poland did not immediately respond to requests for comment about the TVN cameraman.

TVN24 is the most widely watched independent channel in Poland and the network has had run-ins before with the ruling Law and Justice (PiS) party, which has been accused by Brussels of trying to muzzle free media.

“We are treating this as an attempt to intimidate journalists,” TVN said in a statement late on Saturday. “The authors of the report acted in accordance with the standards of investigative journalism.”

The TVN footage showed members of a far-right group dressed in Nazi uniforms, burning a wooden swastika, making Nazi salutes and sharing a cake with a symbol of a swastika made of chocolate cookies.

Poland’s broadcasting regulator KRRiT fined TVN last year over its coverage of protests in Poland’s parliament, saying it was “promoting illegal activities and encouraging behaviour that threatens security”.

The U.S. government said the regulator’s actions undermined press freedom and KRRiT reversed the decision in January.

Reporting by Joanna Plucinska and Pawel Sobczak; editing by David Clarke